Minetest is a free and open source infinite-world block sandbox game and a game engine, inspired by InfiniMiner, Minecraft and the like. It has been in development and use since October
It is Free and Open Source Software, released under the LGPL. This way it is more readily available to Linux users, and random people (you!) can fix bugs and experiment effectively.
Minetest is developed by Perttu "celeron55" Ahola and a number of contributors.
Minetest is technically simple, stable and portable. It is lightweight enough to run on fairly old hardware. It currently runs playably on a laptop with Intel 945GM graphics. Though, as for the CPU, dualcore is recommended.